The nervous system of the Mollusks is re markable for the peculiar and distinct colour of the ganglions in certain species, as the fresh water Muscles and Unios; and for the contrast between the density of the cellular sheath of the nerves and the semifluid pulp which it contains. This structure allows of an artificial injection of the nerves, which has led some anatomists to describe them as parts of an absorbent system.
Although the bivalve Mollusks are headless and without brain, some of them have mani fested signs of a perception of light, and in a few species simple ocelli have been detected on the verge of the mantle.
In the Encephalous Mollusks the eyes, when present, are small, never exceed two in number, and are usually supported on flexible peduncles or tentacula; but in the Dibran chiate Cephalopods they are large, always ses sile, and highly complicated.
The organ of hearing is peculiar to the Ce phalopods in the present division of the animal kingdom.
The organ of smell, as a special and cir cumscribed part, has likewise been recognized only in the highest class : but Cuvier observes, that the skin of the Mollusks so clearly re sembles in its softness and lubricity a pituitary membrane, that they probably may recognize odours at every point of their external surface. Professor De Plainville conjectures that in the Gastropods the soft extremities of the first pair of cephalic tentacles may be the seat of the organ of smell. An organ of taste has of course been recognized only in the Encephalous Mollusks, in many of which the tongue is large and complex, and in the Cephalopods is evi dently provided with gustatory papillm. The sense of touch is in this as in other groups of animals the most generally possessed and the most widely diffused over the body. The mar ginal fringe of tentacles on the mantle of most Bivalves ; the branched processes of the skin in certain Pteropods and Gastropods ; the ce phalic tentacles or ' horns,' as they are termed, in the Snail and allied Mollusks; the rich and varied apparatus of cephalic and labial ten tacles—nearly one hundred in number—in the Nautilus ; and the extremities of the acetabuli ferous arms of the higher Cephalopods, may all be regarded as organs of delicate sensa tion : the same opinion may reasonably be entertained of the highly vascular surface of the ventral locomotive disc or foot' in the Slug, Snail, and other Gastropods.
The voluntary muscular fibre of the Mollus cons animals is distinguished from that of the Articulate and Vertebrate animals by the ab sence of the transverse stria, and in most of the Acephalous Mollusks it is antagonized by a merely elastic gelatinous fibre. In the Tuni caries the flexible outer coat obeys and opposes the change of form which the inner muscular envelope occasions. In the Bivalves, whose shell affords firm levers of attachment to the muscles, the antagonizing elastic force is im planted at the hinge of the shell; and some of the species (11follusca subsilentia of Poli) can, by virtue of this mechanism of solid lever with its attached muscular and elastic fibre, execute short leaps.
The Encephalous Mollusks with a cerebral centre of nervous influence antagonize one series of muscles by the regulated action of another series, and are no longer dependent on mere elastic fibres for their movements. The musculo-cutaneous mantle is produced in the form of fins in the Pteropods, Heteropods, and some Cephalopods ; or there is an accumula tion of longitudinal fibres, intersected by ob lique or transverse ones on the ventral surface of the mantle, producing the thick contractile disc, which is termed the foot. This some times extends the whole length of the body, as in the Gastropods, sometimes is developed only from the region of the neck, as in the Trache lipods. The attachment of the body to the shell in these Mollusks, the presence of, and power of retracting and elongating, a siphon or breath ing tube, the movements of the head and its appendages, especially when these are deve loped into instruments of progressive motion and prehension, as in the Cephalopods, are the chief conditions of the progressive advance ment and complication of the muscular system in the Molluscous sub-kingdom.
The heterogangliate type of the nervous sys tem, with the correspondingly low condition of the muscular and other organs of relation, which, commencing in the Ciliobrachiate Po lypes, is established in the Mollusks, is on the whole very inferior to the conditions and powers of the sensitive and motive systems in the Arti culates : but, on the other hand, " the organs of growth and reproduction become more evolved ; and in the Mollusca we are presented with a perfecting of the internal organs, which is to prepare for and to be more fully deve loped in the higher animals." The alimentary canal is provided with a se parate mouth and vent : the stomach may be distinguished from the cesophagus and intes tine, and a liver is present in all the Mollusks, and is remarkable in most for its large size and complicated lobular form. The bile is secreted from arterial blood. The mouth in the Ace phalous Mollusks is generally concealed in the interior of the pallial cavity, but is always si tuated at the anterior part of the body, or that which is opposite to the excrementory and re spiratory tubes or orifices. There are neither jaws nor salivary glands in this division. Among the Encephala, however, in which the mouth is armed with horny plates representing in diffe rent species the knife, the saw, the rasp, or the seisms, the salivary system attains an extraordi nary degree of development, especially in the Cephalopods, in which the mandibles resemble those of the Raptorial bird, and sometimes have their margins armed with a calcareous dentated plate. In the Cephalopods the pancreas makes its first appearance. The special forms of the progressive complications of the digestive sys tem in the present division of animals will be found amply illustated in the articles TUN I